1. Field of the Invention
The present invention is directed to catalytic dewaxing of petroleum chargestocks over a molecular sieve catalyst composition having an inert binder of large pore size, the preparation of the catalyst composition, and the catalyst composition.
2. Description of the Prior Art
Lube base stock oils are derived from various crude oil stocks by a variety of refining processes directed toward obtaining a lubricant base stock of suitable boiling point, viscosity index (VI), cloudpoint, overnight clouding and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers. The distillation provides one or more raw stocks within the boiling range of about 450.degree. F. to 1010.degree. F. (232.degree..varies.566.degree. C.). The raw stocks are subjected to the separation of undesirable aromatic components and finally, to dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, the use of asphaltic type crudes is not preferred as they contain large quantities of aromatic components and yield extremely low levels of acceptable lube stocks. Paraffinic and naphthenic crude stocks are preferred but aromatic separation procedures will still be necessary to remove aromatics. In the case of lubricant distillate fractions, generally referred to as the neutrals, e.g., heavy neutral, light neutral, etc. the aromatics will be extracted by solvent extraction using a solvent such as furfural, n-methyl-2-pyrrolidone, phenol or other material which is selective for the extraction of the aromatic components. The residue recovered from such a solvent extraction of aromatics is called a raffinate. The raffinate is relatively free of aromatics and therefore has improved viscosity indices, but still contains paraffins which adversely effect the pour point and other properties. With the heavier residuum from the lower portion of the vacuum tower (short residuum), asphaltenes will first be removed in a deasphalting step, e.g., with propane, followed by solvent extraction of residual aromatics to produce a heavy raffinate generally referred to as bright stock. In either case, with lighter raffinates or bright stock, a further catalytic dewaxing step is normally necessary to reduce waxy paraffins in order for the lubricant to have a satisfactorily low pour point and cloud point, so that the lubricant will not solidify or precipitate under low temperature conditions. The term dewaxing means the removal of those hydrocarbons (waxes) which will readily solidify.
Dewaxing has been carried out both catalytically and with solvents. In solvent dewaxing, a solvent such as a mixture of methyl ethyl ketone (MEK) and toluene or liquid propane is used, followed by chilling to induce crystallization of the paraffin waxes for removal.
Catalytic dewaxing processes are described, for example, in U.S. Pat. Nos. 3,700,585, Re. 28,398, 3,956,102 and 3,968,024. A subsequent hydrotreating step may be used to stabilize the product by saturating lube boiling range olefins produced by the selective cracking which takes place during catalytic dewaxing. Reference is made to U.S. Pat. Nos. 4,181,598 and 4,437,975 for descriptions of such processes.
A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. Processes of this type have become commercially available as shown by the 1986 Refining Process Handbook, page 90, Hydrocarbon Processing, September 1986, which refers to the availability of the Mobil Lube Dewaxing Process (MLDW). The MLDW process is also described in Chen et al., "Industrial Application of Shape-Selective Catalysis" Catal. Rev. Sci. Eng. 28, (283), 185-264 (1986), especially pp. 241-247, to which reference is made for a further description of the process. Reference is made to these disclosures for a description of various catalytic dewaxing processes. Catalytic dewaxing processes generally utilize ZSM-5 type catalysts.
Generally, light raffinates dewaxed with ZSM-5 catalysts suffer some losses in yield and viscosity indices relative to solvent dewaxing to identical pour points. Zeolites with more constrained pores and therefore greater selectivity such as zeolites from the ferrierite family, i.e., ZSM-22, 23, 35, 57 and 58, have been used to recapture some of these losses. Processes of this type are described, for example, in U.S. Pat. Nos. 4,222,855, 4,372,839, 4,414,097, 4,524,232 and 4,605,888. Although some of these more constrained catalysts perform relatively well with light hydroprocessed feeds, they typically have difficulty in or are incapable of processing non-hydroprocessed and even heavier hydroprocessed feeds.
In addition, in the dewaxing of heavy raffinates such as bright stock, the presence of large waxy naphthenic-type molecules (cycloparaffins) cause hazing which results from the formation of microcrystalline wax particles that can occur over time at low storage temperatures in the range of the pour point of the stock. Haze prevention in lubricant basestocks and products is desired for appearance as well as the engineering function of insuring good low temperature pumpability and filterability in certain lubrication systems, especially in systems where fine filtration is required for maintaining critical lubricant cleanliness. The naphthenic-type molecules involved in haze formation occur naturally in petroleum.
Dewaxing of lubricant basestocks removes much of these troublesome components, especially solvent dewaxing, e.g., with methyl ethyl ketone (MEK) and toluene. In catalytic dewaxing of heavy raffinates, however, these components are not easily removed and they can be left behind in the basestock. For this reason, catalytically dewaxed bright stock raffinate basestocks suffer poorer low temperature hazing characteristics relative to basestocks processed to similar pour point through solvent dewaxing. In order to mitigate this problem, catalytic conversion to lower point is practiced. This, however, results in lower basestock yields and shorter process cycles (faster catalyst aging) in catalytic dewaxing. With some particularly troublesome feedstocks, this problem cannot always be easily or economically remedied with current catalytic dewaxing technology.
Zeolite catalysts have often been incorporated with a matrix or binder material to impart strength during hydrocarbon conversion processes. The most commonly used matrix materials include alumina, clay and amorphous silica derived from inorganic sources. Binder materials may contribute chemical properties such as acidity and physical properties such as surface area and high or low density. The aluminas may have activity; for example, gamma alumina has Lewis acid sites and Bronsted acidity. Amorphous silica, on the other hand, has low activity. Silica gel is three-dimensional network of particles of colloidal silica and may be of regular, intermediate or low density. The hydrous clays are generally chemically inactive but some are chemically modified for activity.
The use of a steamed porous silica gel as a support is described in U.S. Pat. No. 3,369,274. U.S. Pat. No. 4,582,815 describes a catalyst produced by mulling silica, a zeolite, water and a base such as sodium hydroxide. U.S. Pat. No. 5,182,242 describes extruding zeolite, low activity refractory oxide binder such as silica wherein the silica is derived from an inorganic silica rich solid such as amorphorous silica or hydrated silica in which the silica concentration is at least 50%.
None of the binder materials previously described encompasses an inert binder of large pore size nor an organic silicon source for the binder.